add scons helper

Library/Homebrew/utils.rb

@@ -148,6 +148,14 @@ def curl *args
   safe_system curl, *args
 end
 
+# Run scons using a Homebrew-installed version, instead of whatever
+# is in the user's PATH
+def scons *args
+  scons = Formulary.factory("scons").opt_prefix/'bin/scons'
+  raise "#{scons} is not executable" unless scons.exist? and scons.executable?
+  safe_system scons, *args
+end
+
 def puts_columns items, star_items=[]
   return if items.empty?